Freddie Roach has trained Amir Khan for Saturday night’s bout against Lamont Peterson in D.C. (Harry How/GETTY IMAGES)

Imagine you were there 25 years ago, the last time the rugged little tank climbed through the ropes in Washington. And you saw him swap hooks and crosses with a good fighter named Darryl Tyson, until he lost a 12-round war for the Continental Americas lightweight title.

You saw his will, his heart.

And after a black fan kissed him on the forehead that night, telling him, excitedly, “You’re the toughest white [expletive] I ever saw,” imagine all you heard later about that fighter was that he developed Parkinson’s disease at just 28 years old.

That’s it, right — boxing’s cliched ending, nothing to something and back to nothing again?

Except Freddie Roach flipped the script. Somehow, his story began after he unlaced his gloves for the final time — eventually.

“For a while I did go back to nothing,” the famed fight trainer said Thursday morning as he sipped from a Starbucks cup in his hotel. “If Eddie [Futch] hadn’t asked me to help out Virgil Hill because he was so busy training other fighters, I really don’t know where I’d be. I was a telemarketer, selling pens and key tags. My name was Joe Davis.”

In 53 pro fights, Roach biggest payday was $7,500. For one recent Manny Pacquiao fight, he made about $1 million. “I guess I found something I could do better than boxing,” Roach said, snickering.

For all the big names in the fight game Saturday night at the Walter E. Washington Convention Center, where Roach-trained Amir Khan vs. homegrown Lamont Peterson headlines the District’s first major championship card in nearly two decades, none is suddenly more boffo box office than Freddie Roach, who was elected to the International Boxing Hall of Fame this week.

The gym Mickey Rourke helped him start in the early 1990s, the Wild Card Boxing Club in Hollywood, Calif., has become a shrine to the sport the way Emmanuel Steward’s Kronk Gym in Detroit once was. Roach is the subject of an HBO series, “On Freddie Roach,” next month, in which the network’s cameras, reality-show style, follow the trainer of 31 world champions — none more famous than Pacquiao. The series is said to be a more layered, complex “24/7.”

“One thing: I didn’t realize I shake as much I do,” Roach, 51, said. “That’s embarrassing sometimes to watch. When my mind is somewhere else, I shake a lot because I’m so focused on that thing. When I come back to where I am, I can stop shaking if I know I’m shaking.”

He takes medication three times a day, including in the afternoon when the Parkinson’s tremors sometimes increase. He knows the disease is progressive. Having received the diagnosis in 1992, Roach also now sees it as part of him, something he can even use humor to fight.

“I’m at a buffet one time and I’m having trouble getting a piece of chicken on the spoon, right,” he begins. “And I got a plate full of food in this hand, but my mind is on the chicken in the other hand. By the time I got the chicken on the spoon, my plate was empty.”

As rewarding as it has been training Pacquiao — who Roach believes has a “better than 60 percent chance” of facing Floyd Mayweather next spring in a fight boxing needs like no other — Roach beams when he talks about a day nearly 10 years ago at his gym.

Unannounced, the most famous Parkinson’s patient of all walked into the Wild Card. And for four majestic hours, Muhammad Ali underwent a startling transformation.

“His symptoms disappeared, they were gone — he just got calm as he began hitting the bag,” Roach said. “It was the greatest day in the history of the gym. He did magic tricks. He levitated for us.”

As Ali gloved up and Roach danced around the ring with him, the sublime choreography so familiar two decades after their retirements returned. Roach’s symptoms were gone too.

Such a bizarre juxtaposition, no? The cruel sport responsible for their awful afflictions gave both men this moment of peace.

Even now, once Roach puts the mitts on and begins catching punches, it becomes almost oddly meditative for him — calming, often removing the tremors until he leaves the ring.

“I can’t explain it, but it’s such a natural thing — I don’t know what else I’d do,” Roach said.

Today, he works with 14 fighters between 8 a.m. to 8 p.m. at the Wild Card, and has gone as many as 67 rounds during one non-stop session — essentially more than three hours of catching punches from some of the most physically dangerous fighters in the world. “Including the last 15 rounds with Manny,” he said.

As he finishes his coffee, the little tank, more jacked in the biceps now than at his fighting weight, smiles. He fidgets for a moment, his hand momentarily shakes, and he again thanks Eddie Futch, the late legendary trainer, for his start. Roach also recounts his last fight and the man who won’t be at his induction ceremony.

Paul Roach, the violent man who raised three fighters and was once himself a former New England featherweight champ, trained Freddie at the end of his career.

“What happened?” he asked his son in the dressing room after losing a 10-round decision in his last fight.

“What do you mean?” Freddie replied.

“How could you have been so good and end up like this?”

“Go [expletive] yourself,” Freddie shot back.

That was 25 years ago. He only saw his father once after that — when he was suffering from Alzheimer’s and didn’t know who Freddie was.

“When me and my brothers retired from boxing, he kind of gave up on life after that,” Roach said. “I wish I didn’t say that to him. But I know he would be proud of me now. He loved boxing. He’d be here with me; I know it.”

Wherever Paul Roach is now, he should know: His kid made more of himself than almost anyone in this unforgiving business. Freddie Roach turned the end into the beginning.

What happened? He didn’t merely become good; he became the best. That’s what happened.